1. Field of the Invention
This invention relates to an FSK data demodulator for demodulating FSK (frequency shift keying) data and particularly to an FSK data demodulator for demodulating FSK data with a converting circuit for converting a transmitted FSK signal into a baseband signal directly.
2. Description of the Prior Art
An FSK data demodulator is known. Such FSK data demodulator demodulates a received FSK data signal such that it obtains quadrature baseband signals and it decodes the quadrature baseband signal to a data signal. It obtains the baseband signal by direct conversion. That is, the baseband signals are directly converted from the received data signal.
Such a prior art FSK demodulator is disclosed in Japanese patent application provisional publication No. 58-19038. FIG. 20 is a block diagram of such a prior art FSK demodulator.
Hereinbelow will be simply described a prior art FSK data demodulator with reference to FIG. 20.
In FIG. 20, an FSK received signal is supplied to a mixer 81 and to a mixer 82 through a ninety-degree phase shifter 83 at the same time. The received signal and an output of the ninety-degree phase shifter 83 are down-converted by mixing them with an output signal of a local oscillator 84 respectively, and then, passed through low pass filters 85 and 86 for passing baseband signals to obtain I and Q signals respectively. The I signal is formed into a digital signal by an amplitude limiting amplifier 88. The Q signal is subjected to phase shifting by a ninety-degree phase shifter 87, and then, is formed into a digital signal by an amplitude limiting amplifier 89. Data is decoded by a logic operation circuit 90.
The reason why the direct conversion is adopted is that it is easy to implement its circuit in an IC because the circuit structure is simple. However, if communication speed is high such that a symbol rate of a transmitted data is equal to or higher than an FSK phase shift amount, there is a problem that the ninety-degree phase shifter should have an extremely broad band because it is necessary to shift a phase of the received data signal from a low frequency to a frequency of an order obtained by a sum of the modulation frequency shifting and the symbol rate to effect accurate decoding. Generally, a ninety-degree phase shifter capable of phase shifting a signal of a low frequency has a difficulty in implementation of its circuit into an integrated circuit (IC) because it needs a large capacitor in the circuit configuration. Thus, it prevents saving power consumption and miniaturization.
Moreover, in the actual receiver including the prior art FSK demodulator, there is a problem that if a high speed FSK signal mentioned above is received, demodulation is difficult because the signal to be phase-shifted contains a lot of discontinuous points, so that phase shifting becomes imperfect by the ninety-degree phase shifter and is difficult in demodulation.